1. Field of the Invention
This invention generally relates to communication networks and protocols, with particular relevance to wireless networks, or other networks requiring minimal turnaround time. It specifically addresses a control protocol for wireless ATM, although the features presented can be applied to alternative protocols as well.
2. Discussion of the Related Art
Currently, communication networks are formed by interconnecting devices by wire or cable, and having each device conform to a protocol for sending messages along these wires and cables. In some instances, a portion of such a network may be implemented as a wireless connection, employing radio or infrared frequency signals between nodes. Such wireless connections are point-to-point, having a single communications device at each end, each tuned to each other at a frequency different from other devices in the same geographic area.
A wireless network, on the other hand, is formed without physical connections among the devices, employing, for example, radio frequency signals. Each device on the network is tuned to the same frequency, and each device conforms to a protocol for sending messages at this common frequency. The protocol may allow communication among all the devices in the network or the protocol may constrain each device to only communicate with a master device. Wireless networks offer a significant logistical advantage over wired networks, by obviating the need to run wires or cables to each device.
With increased availability of multimedia technologies, and the increased demand for information access, the market potential for residence or business based Local Area Networks (LANs) is growing. The ease of installation and expansion of a wireless network is certain to create a large demand for wireless LANs. For example, a central base station may provide wireless services, including voice, video, and data, to all the communications devices in one's home, or a wireless base station may provide for the communication among all the portable computers in an office, or all the computers on a campus. To be successful, however, the techniques and protocols employed in these wireless networks must not be significantly inferior to their wired network equivalents, and they must be able to provide compatibility with existing networks, particularly in terms of compatible service and management criteria.
During the past decades, protocols have been developed for effectively and efficiently managing the transfer of information within networks of communicating equipment. An underlying premise in the development of these network protocols has been that of a wired network infrastructure. In a wireless network, the assumptions upon which the wired network protocols were developed may no longer be valid. Although most of the existing protocols are functionally extensible to wireless networks, their effectiveness and efficiency may be adversely affected by the lack of a direct connection among devices.
Additionally, associated with a wireless network is the likelihood of the presence of portable, battery powered, devices. Although some wireless devices, such as desktop computers, televisions, and home theatre systems will be powered by AC line supplies, a number of wireless devices, such as telephones, cameras, and laptop computers will be powered by batteries. In providing for a wireless network protocol, consideration must be given to an architecture which allows for power conservation as well. Such considerations are rarely given to wired networks.
A common protocol employed for data communications in a wired network is Asynchronous Transfer Mode (ATM). ATM has been developed to deal with high speed data with different data rates, different quality of service (QoS) requirements (for example, data reliability, delay considerations, etc.), and different connection or connectionless paradigms for multimedia communications. ATM is well suited for multiplexing video, audio, and data in the same medium, by proper choice of the parameters required for each. Audio data, for example, does not require the packet-error reliability reburied of data services, but cannot tolerate excessive delay. Video data can in general suffer more delay than audio, but is intolerant to delay jitter. These delay and packet-error rate considerations are best supported by a connection oriented service, wherein the parameters are negotiated and established at the commencement of each connection. For optimum performance, ATM adopted an end-to-end error detection approach, premised on the assumption that the error rate associated with the communications medium, such as wired fiber-optics, was minimal. Only the terminal equipment monitors for errors; if an error is detected, a retransmission request is sent to the originating transmitter. ATM also blocks any services for which it cannot guarantee the required QoS. These characteristics, and other facets of ATM, are very effective for multimedia communication on a wired network, but are the very factors which are contrary to existing wireless network characteristics.
Existing efforts of building a wireless local area network (LAN) are focussed on emerging standards, such as IEEE 802.11 in the United States, and HIPERLAN in Europe. These standards do not take into consideration the ATM based quality of service (QoS) requirements for real time and data traffic, particularly in the area of delay. In a typical wireless network, delays will increase exponentially in an overloaded network, as each transmitter contends for access. Most wireless networks operate using some form of collision detection and receipt acknowledgement protocol. For example, each transmitter will listen for a quiet period, then transmit a packet. If another transmitter does not transmit at the same time, the receiver will receive the packet and acknowledge the receipt to the transmitter. If another transmitter transmits simultaneously, however, a collision occurs, the intended receiver(s) receive garbled messages, and no acknowledgement is sent. Upon the non-receipt of an acknowledgement, each transmitter will again attempt to transmit, hopefully not simultaneously. Wireless networks are characterized, in general, as having higher error rates, unpredictable delays, and requiring intermediate error detection and correction. Thus, typical wireless networks are inherently unsuitable for ATM traffic.
Clearly, an important issue in designing a wireless ATM is that the control protocol which specifies the method of access among multiple users to the same medium, the Medium Access Control (MAC) protocol, must satisfy the basic requirements of ATM, particularly in the area of delay considerations. One such protocol is the Dynamic Slot Assignment (DSA++) protocol designed for use within a European project, Mobile Broadband System (MBS). This protocol was premised, however, on the assumption that the uplink and downlink communication paths, to and from a base station, were each effected on different frequencies. This reduced the turnaround time for control and acknowledgements, but required all stations to accommodate transmission and reception on two discrete and non interfering frequencies.
Thus, it is seen that the transformation of a wired network protocol, such as ATM, to a wireless network protocol typically requires additional time, or frequency, or both. It is the purpose of this invention to minimize the time required to communicate information within a network, without requiring additional frequency allocation, while supporting the QoS concepts of ATM. Although the invention presented is particularly applicable to wireless ATM networks, the principles embodied are equally applicable to minimize the time required to transfer information on other wired or wireless networks as well.